Re: HVM Migration of domU on Qemu-upstream DM causes stuck system clock with ACPI

["Roger Pau Monné" <roger.pau@citrix.com>]

On 03/06/13 12:05, Stefano Stabellini wrote:
> On Mon, 3 Jun 2013, Roger Pau Monné wrote:
>> On 31/05/13 17:10, Roger Pau Monné wrote:
>>> On 31/05/13 15:07, George Dunlap wrote:
>>>> On 31/05/13 13:40, Ian Campbell wrote:
>>>>> On Fri, 2013-05-31 at 12:57 +0100, Alex Bligh wrote:
>>>>>> --On 31 May 2013 12:49:18 +0100 George Dunlap
>>>>>> <george.dunlap@eu.citrix.com>
>>>>>> wrote:
>>>>>>
>>>>>>> No -- Linux is asking, "Can you give me an alarm in 5ns?"  And Xen is
>>>>>>> saying, "No".  So Linux is saying, "OK, how about 5us?  10us?
>>>>>>> 20us?"  By
>>>>>>> the time it reaches 4ms, Linux has had enough, and says, "If this timer
>>>>>>> is so bad that it can't give me an event within 4ms it just won't use
>>>>>>> timers at all, thank you very much."
>>>>>>>
>>>>>>> The problem appears to be that Linux thinks it's asking for
>>>>>>> something in
>>>>>>> the future, but is actually asking for something in the past.  It must
>>>>>>> look at its watch just before the final domain pause, and then asks for
>>>>>>> the time just after the migration resumes on the other side.  So it
>>>>>>> doesn't realize that 10ms (or something) has already passed, and that
>>>>>>> it's actually asking for a timer in the past.  The Xen timer driver in
>>>>>>> Linux specifically asks Xen for times set in the past to return an
>>>>>>> error.
>>>>>>> Xen is returning an error because the time is in the past, Linux thinks
>>>>>>> it's getting an error because the time is too close in the future and
>>>>>>> tries asking a little further away.
>>>>>>>
>>>>>>> Unfortunately I think this is something which needs to be fixed on the
>>>>>>> Linux side; I don't really see how we can work around it in Xen.
>>>>>> I don't think fixing it only on the Linux side is a great idea, not
>>>>>> least
>>>>>> as it makes any current Linux image not live migrateable reliably.
>>>>>> That's
>>>>>> pretty horrible.
>>>>> Ultimately though a guest bug is a guest bug, we don't really want to be
>>>>> filling the hypervisor with lots of quirky exceptions to interfaces in
>>>>> order to work around them, otherwise where does it end?
>>>>>
>>>>> A kernel side fix can be pushed to the distros fairly aggressively (it's
>>>>> mostly just a case of getting an upstream stable backport then filing
>>>>> bugs with the main ones, we've done it before) and for users upgrading
>>>>> the kernel via the distros is really not so hard and mostly reuses the
>>>>> process they must have in place for guest kernel security updates and
>>>>> other important kernel bugs anyway.
>>>>
>>>> In any case, it seems I was wrong -- Linux does "look at its watch"
>>>> every time it asks.
>>>>
>>>> The generic timer interface is "set me a timer N nanoseconds in the
>>>> future"; the Xen timer implementation executes
>>>> pvclock_clocksource_read() and adds the delta.  So it may well actually
>>>> be a bug in Xen.
>>>>
>>>> Stand by for further investigation...
>>
>> I've been investigating further during the weekend, and although I'm not
>> familiar with the timer code in Xen, I think the problem comes from the
>> fact that in __update_vcpu_system_time when Xen detects that the guest
>> is using a vtsc it adds offsets to the time passed to the guest, while
>> in VCPUOP_set_singleshot_timer Xen compares the time passed from the
>> guest using NOW(), which is just the Xen uptime, without taking into
>> account any offsets.
>>
>> This only happens after migration because Xen automatically switches to
>> vtsc when it detects that the guest has been migrated. I'm currently
>> setting up a Linux PVHVM on shared storage to perform some testing, but
>> one possible solution might be to add tsc_mode="native_paravirt" to the
>> PVHVM config file, and another one would be fixing
>> VCPUOP_set_singleshot_timer to take into account the vtsc offsets and
>> correctly translate the time passed from the guest.
> 
> Good analisys!
> I think that the right solution would be to fix
> VCPUOP_set_singleshot_timer.

As a band aid I can confirm that adding tsc_mode="native_paravirt" seems
to be working fine (with the tests I've done so far), but it requires
the admin to know whether a certain HVM will be using the PV timer or
not, which I guess it's not possible in every case.

Xen could also force the TSC mode to native_paravirt when it detects
that a HVM guest is using the PV timer, but I don't think that's the
right approach. Is this something we aim to fix before the 4.3 release?


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